You will need to poison them, so you'll have to think of something you can fix over the treated stems to keep the kittens off. Cut the plants down as low as possible - if they're an inch across as you say, drill into the centre with a medium size woodbit to make a hole about quarter inch deep, fill the hole with SBK, a brushwood killer (preferably without spilling it, use it neat, but it's like water). Then cover with something.